Method of supervising skin thickness in a solidifying body such as a continuously cast ingot

ABSTRACT

An early indication of skin separation and expected thinning of the skin in a mold for continuous casting is provided by detecting the ratio of heat flow into the mold in two zones of limited width of the mold wall, one above the other and located where the skin is most likely to begin to separate. The resulting parameter K in conjunction with total heat flow Q into the zone is related to skin thickness s, generally, by the relation   wherein A&#39;&#39;, B&#39;&#39; and C&#39;&#39; are constant parameters for the mold when operated at a constant casting speed. The total heat flow Q into the two zones is either determined directly by measuring temperature and coolant flow or indirectly by the relation   wherein A, B and C are additional parameters and To is the measured surface temperature of the ingot at the mold bottom opening. In the preferred form, the heat flow ratio is determined by temperature measurements in one cooling duct, traversing the mold side vertically.

United States Patent [1 1 Dorr et a1.

[ Dec.2, 1975 [75] Inventors: Wolfgang Diirr, Essen; Hartwig Matznor;Tilman Noska, both of Duisburg, all of Germany [73] Assignee: MannesmannAktiengesellschaft,

Dusseldorf, Germany [22] Filed: Apr. 1, 1974 [21] Appl'. No.: 456,899

[30] Foreign Application Priority Data Apr 17, 1973 Germany 2320277 [52]U.S. Cl. 164/4; 73/295; 164/154 [51] Int. Cl. B22D 11/12 [58] Field ofSearch 164/4, 82, 154, 283 M; 73/295 [56] References Cited UNITED STATESPATENTS 3,145,567 8/1964 Bobrowsky 73/295 3,204,460 9/1965 Milnes 164/4UX 3,478,808 11/1969 Adams. 164/82 X 3,502,133 3/1970 Carson. [64/154 X3,745,828 7/1973 Howellm, 73/295 3,797,310 3/1974 Babcock et al. 164/154X FOREIGN PATENTS OR APPLICATIONS 1,029,098 3/1953 France 164/154Primary Examiner-R. Spencer Annear Attorney, Agent, or FirmRalf H.Siegemund [57] ABSTRACT An early indication of skin separation andexpected thinning of the skin in a mold for continuous casting isprovided by detecting the ratio of heat flow into the mold in two zonesof limited width of the mold wall. one above the other and located wherethe skin is most likely to begin to separate. The resulting parameter Kin conjunction with total heat flow 0 into the zone is related to skinthickness 5, generally, by the relation wherein A, B and C are constantparameters for the mold when operated at a constant casting speed. Thetotal heat flow Q into the two zones is either determined directly bymeasuring temperature and coolant flow or indirectly by the relation logQ B wherein A, B and C are additional parameters and T is the measuredsurface temperature of the ingot at the mold bottom opening. In thepreferred form. the heat flow ratio is determined by temperaturemeasurements in one cooling duct. traversing the mold side vertically.

12 Claims, 4 Drawing Figures (457m; awzeaz I U.S. Patent Dec.2, 1975 'sheetzf (4577M; (ax rial METHOD OF SUPERVISING SKIN THICKNESS IN ASOLIDIITYING BODY SUCH AS A CONTINUOUSLY CAST INGOT BACKGROUND OF THEINVENTION The present invention relates to a method and equipment forascertaining, supervising and controlling the thickness of the skin of asolidifying body. More particularly but not exclusively the presentinvention relates to the supervision of the growth of the skin of acontinuously cast ingot as extracted from a liquid cooled mold which isopen at top and bottom.

As is well known, particularly from continuous casting of steel, theingot as withdrawn from a mold is by no means solidified all the waythrough. Rather, the ingot has a liquidous core which extends inside ofthe ingot for a distance much longer than the height of the mold; theskin is still quite thin at the bottom of the mold. A significant aspectof continuous casting is to be seen in the desirability of obtaining askin of adequate and uniform thickness over the entirecircumferences,right at the bottom exit of the mold so that thewithdrawn ingot has at least some strength at that point. Thisparticular area is most critical and disturbances are most likely tooccur here unless duly prevented.

Adequate skin thickness at the mold bottom is obtained by appropriatelyproportioning the interrelating casting speed, rate of cooling and moldcontour (e.g. conicity). Empirical data have been acquired here overlong periods of time devoted to experiments and practice, and as aresult one can predetermine these parameters today so as to obtain anadequate skin thickness with sufficient certainty, whereby adequate isto mean a skin thickness that will hold and will not rupture or thelike, with a considerable margin of safety.

It has to be observed, however, that even today ones does not know withsufficient certainty all of the details of the phenomena and events thatactually transpire inside of a mold. The interior of a mold e.g. forcontinuous casting of steel is not readily accessible for taking directmeasurements. It is believed, therefor, that the presently usedwithdrawal and casting speeds are low primarily for reasons of operatingwith a "sure margin of safety, but such caution may well be needlesslyexcessive in cases. Lack of data and information has, therefor,precluded increasing the through-put of molds.

It is known to measure the skin thickness outside of the mold, justunderneath its bottom opening, by transmitting radiation transverse toand through the ingot so as to detect skin thickness (vs. liquidous corethickness) and to control the casting speed in dependence thereof.However, one can ascertain the skin thickness below the mold only i.e.,at an instant when it may already be too late.

In accordance with another approach the problem is simulated on thebasis of a mathematical model by means of which one can attempt toinvestigate mathematically the processes and phenomena inside of themold. In a kind of iteration and/or approximation process one canattempt here to gain information on the inside events using theexternally accessible data as boundary conditions. It is possible inthis manner to obtain some information on the limitations of the castingprocess as to speed.

The largest unknown" in that model is, however, the local formation of agap, limiting the accuracy in the determination of the heat transferinto the mold as a function of distance from the bottom. That heattransfer is not constant over the mold height to begin with, andsuddenly occurring local changes in the heat transfer conitions due tolocal separation of the skin from the mold wall are simply not amenableto calculatory prediction. Presently, one simply does not know withsufficient accuracy when the skin will separate from the mold wall atany particular spot (and not elsewhere and not earlier or later) becausethe modus operandi of that phenomenon is not sufficiently known eventhough one does know that certain mold wall regions are more likely thanothers to develop skin separation. Thus, the separation appears tooccur, as far as is known at this time, more or less at random. Thisaspect is only one of the problems; the heat transfer in and throughsuch a gap is quite a complex phenomenon particularly when compared withthe heat transfer above the gap as well as when compared with the heattransfer as it is effective prior to gap formation is a local phenomenawhich does not occur (or only rarely) simultaneously over the entirecircumference of the forming ingot, nor will ,the gap form in the samelevel in the mold in all cases.

Of course, some measurements can be made with regard to the process. Forexample, one can measure the heat through-put of the mold, possibly inrelation to poured-in molten steel per unit time and for a temperatureof the molten material which can likewise be measured, at least on thesurface. The thermal energy is specifically measured by determining theamount of water passing through the mold (per unit time), and bydetermining its temperature differential at input and output. The datagained here become more meaningful if measured and recorded separatelyfor the different sides of the mold. On the basis of data gained here,variations in removed thermal energy can be used for control of thecasting process.

However, one can see that the heat flow and transfer measured in thatmanner yields a more or less summary kind of data, integrated for entiremold sides. It is simply not possible to learn sufficient details as toany local conditions, particularly with regard to portions of the moldwhich are, for example, specifically endangered or prone to exhibitlocal (and then progressing) gap formation. Total thermal input andvariations thereof as measured for the entire mold, or even for eachside separately, is simply not adequate information which permitslocalization of any trouble spot that may cause (or does cause) skinseparation resulting in a significant thinning of the skin (and,possibly, rupture) when leaving the mold.

DESCRIPTION OF THE INVENTION It is an object of the present invention tosolve the problem broadly outlined above and to avoid the drawbacksinherent the known methods. Particularly, it is an object of the presentinvention to provide for definition of a particular parameter whichpermits the drawing of conclusions about the formation of the skin, itsthickness at the mold bottom exit, and particularly about the skinconfiguration at those mold areas which establish most likely troublespots as has become known empirically (after the fact). Moreover, thisparameter should permit a comparison of the skin thickness in variousportions around the periphery. Finally, this parameter should permitutilization as an input for process control of the continuous castingoperation.

It is a specific object of the present invention to provide for a methodfor determining and supervising the skin thickness of a solidifyingbody, particularly the growth of the skin in a continuously cast ingot,during continuous casting while using a liquid cooled mold which is openat top and bottom, whereby the skin of the ingot is rather thin whenwithdrawn and only gradually thickens so that the total length of theliquidous core is larger than the mold is high.

It is a further object of the present invention to provide for asupervision of the skin growing process of a continuous cast ingot thatyields information on conditions in the mold, even prior to emerginge.g. of a thinner-than-normal skin.

In accordance with the preferred embodiment of the present invention itis suggested to define two different, rather narrow cooling zones in amold wall, extending over a relatively small width as determined incircumferential direction of the mold; the two zones are verticallyseparated. The ratio of the heat flows in the two zones, from the moldwall into the coolant, is detected on a running basis as the primaryparameter K. The total heat transfer Q into the two zones isadditionally determined, either directly or indirectly, and the valuesfor K and Q are used to ascertain the thickness of the skin expected toemerge shortly thereafter from the mold bottom exit. The directdetermination of heat transfer involves the measurements of coolantflow, and of the coolant temperature before and after heat exchange withthe mold wall in both of the two zones. The indirect determination of Qinvolves measuring the surface temperature T of the skin where leavingthe mold and relying on a functional relation between K, Q and T fordetermining Q.

The zones of detection are located in one of those portions of the moldwall which are predominantly prone to exhibit (or produce?) separationof the skin from the mold wall. In accordance with another feature ofthe invention, the supervision and indirect measurement of skinthickness is carried out separately in different areas of the mold, andthe resulting skin thickness values are compared to ascertain uniformityor non-uniformity of the casting process. I

The invention is based on the discovery that the particular parameter Kas defined is a very valuable quantitive representation and indicator ofskin separation. When considering a (hypothetical) horizontally takenslice of the ingot, as it drops down in the mold, such a slice begins tocool in the mold and begins particularly to grow a solid skin adjacentthe mold side as it progresses down in the mold; the heat transfer fromthat slice into the mold wall is not uniform as a function of moldheight (i.e., distance from the bottom exit). The growing skin as such,i.e., the increasing thickness of the skin with reducing distance fromthe mold bottom is one factor for non-uniformity of the heat transfer,but separation when actually occurring is another one, having a ratherpronounced effect on local heat transfer and on the subsequent growth ofthe skin. If one plots heat transfer density over distance (frombottom), there is a definite maximum in the upper mold portion i.e., ata relatively large distance from the bottom exit.

The invention utilizes this phenomenon. By means of simulation in amathematical (computer) model it was found that the (local) skinthickness at the mold exit depends, on the one hand, on the entire heatflow into the mold portion and taken over the width of that mold sectionthat contributed to the skin growth. The skin thickness depends furtheron the relative distribution of heat flow, i.e., on the heat flow intothe mold wall in the upper mold portion in relation to heat flow intothe lower mold portion. This holds true with or without skin separation;the separation results in a material disturbance of these heat flowrelations and is reflected in a reduced skin thickness. Hence, when theonset of such disturbance can be detected, the expected reduction inskin thickness can be deduced from such a detection.

Upon supervising the heat transfer in upper and lower mold portions andin a width section most likely to produce separation, the onset ofachange in parameter K will be detected before the effect of the skinseparation has resulted in a thinner skin at the mold bottom. The skinportion which will be thinner (because of separation) is still inside ofthe mold when the parameter K begins to change. Thus, one learns of theseparation and of the expected thinner skin rather early.

It was further found that the heat transfer distribution and total heattransfer are directly related to the surface temperature of the ingot asemerging from the bottom of the mold. The specific relationship betweenthese parameters will be explained later, but it can be said presentlythat the total heat flow Q from mold to coolant is functionally relatedto the ingot surface temperature so that one can be calculated from theother.

It should be mentioned further that the primary parameter in this methodis K, as variations of K (in time) or more precisely the onset of adeviation of K from a constant value is already a indication that thenormal heat transfer conditions in the supervised zones is disturbed.The parameter Q will also vary in this instance, but changes in K aremore pronounced. Thus, the parameter K can already be used directly forcontrol of the casting process.

The invention uses known mold sidings with vertical cooling channels, aninlet for the coolant in the upper mold portion, usually well above thesurface level of molten material in the mold, and an outlet for thecoolant adjacent the bottom opening. Such a single, vertical coolingduct, for example, is used to define the two cooling zones together,particularly as to their width and location in one mold side. Thetemperatures at the upper and lower ends of the duct yield the neededinformation of total heat transfer in both zones, while the temperaturein the center (vertical) of the duct when measured separately permitsascertaining of heat transfer into upper and lower zones individually;the location of this central temperature measurement defines thedividing line between the two zones. However, the scope of the inventionincludes the provision of separate coolant circulations in upper andlower mold wall portions with inlet and outlet temperatures beingdetermined separately to determine the heat transfer into upper and,lower mold portion. However, for reasons below it is mandatory todetermine also the coolant flow in a two-circulation system, whereas nosuch determination needs to be made in the case of a signal coolantcirculation.

DESCRIPTION OF THE DRAWINGS While the specification concludes withclaims particularly pointing out and distinctly claiming the subjectmatter which is regarded as the invention, it is believed that theinvention, the objects and features of the invention and furtherobjects, features and advantages thereof will be better understood fromthe following description taken in connection with the accompanyingdrawings in which:

FIG. 1 shows a mold, somewhat schematically and in an isometric view fordemonstrating certains aspects of defining mold wall zones;

FIG. 2a and 2b are two related graphs demonstrating functional relationsas discovered and relevant for practicing the inventive method; and

FIG. 3 is a cross-section through a mold siding plus schematicrepresentation of measuring instrumentation and probes for practicingthe inventive method.

Proceeding now to the detailed description of the drawings. FIG. 1 showsa mold in which the inventive method can be practiced and which iscomprised of four sides 1, 2, 3 and 4. The particular mold serves forcasting slab ingots and has wide sides 1 and 2, the narrow sides 3 and 4accordingly. The hatched wall portions refer to those portions of themold adjacent to which most likely the skin may separate first.Particularly, a central area 5 on a narrow mold side, and zones 6 at awide mold side are those where the danger of such separation is mostprevalent.

These zones 5 and 6 are not physical entities in the sense of inserts,but are defined by width sections taken over the entire height of themold. Each such zone is divisible into an upper zone, such as 5a or 6a,and a lower zone such as 5b or 6b with a hypothetical (dashed) dividingline separating the zones.

The particular equipment shown in FIG. 3 can be used in any, some or allof these mold side portions. The mold sides is identified here byreference numeral 7 to indicate that it can be any of the sides of themold. The mold contains predominantly molten steel 17 which iscompletely liquidous in the surface level 17a, while a narrowing,liquidous core 17b remains underneath and for a long distance from themold. Reference numeral 16 denotes the growing solidified skin.

A cooling duct 8 traverses the mold in direction of casting. The moldside area in the immediate vicinity of that vertically running duct canbe regarded as one of the zones 5 or 6. The circumferential width ofsuch a zone is in the essence defined by the width of that mold wall orside portion that is effectively cooled by this one duct 8. The widthdimension of the zone extends transversely to the plance of the drawingof FIG. 3, while a and b in conjunction with the dotted dividing lineare the upper and lower zones to be considered. How the upper zone isactually separated from the lower zone will be described shortly.

A flow meter 9 is disposed in the inflow path for water as flowing intoduct 8 (see arrows) to ascertain and to meter the amount of flow ofcooling water (e.g. per unit time) into the cooling portion of duct 8.The flow meter is, however, optional. Thermo-elements 10, 11 and 12 arestrategically located in duct 8 projecting into the duct with sensingends or points 13. Particularly, element 10 has its sensing pointdisposed at a location where the coolant comes into first contact withwall portions of the duct, closest to the inner mold surface.Thermo-element 11 is located in a central portion of duct 8 and element12 monitors the water temperature where flowing away from the mold sideinterior. The duct may contain baffles, or the like to enhance turbulentflow, particularly right at the thermo-elements where sensing the watertemperature.

As stated, reference numeral 13 denotes for each thermo-element the endpoint as exposed most critically to the coolant water and definedtherewith the point of measurement where the temperature reading istaken. The thermo-element 11 defines the dividing line between upper andlower cooling and mold zones as governed for cooling action by the waterin this particular duct 8.

The thermo-elements furnish electrical output signals representing theinitial coolant temperature T the temperature T in about the center(longitudinally) of the cooling duct and exit temperature T,,,respectively. The temperature differential T T is indicative of theamount of heat transferred from the upper mold zone (a) considered intothe coolant, while the differential T T is indicative of the amount ofheat transferred from the lower mold zone (b) into the coolant. Thetemperature differential T T is proportional to the total heat transferin both zones.

The amount of water and its heat capacity are additional parameters fordetermining these heats Qu, Qe and Q respectively. However, the ratioQu/Qe does not contain the amount of water flow and its heat capacity E.That ratio is merely equal to (1",, T )/(T T A pyrometer 14 may bedisposed under the mold, directly adjacent the withdrawing ingot whereemerging from the mold. That instrument provides a measuring valuerepresenting the casting ingot surface temperature T at the mold exit.

Pyrometer 14 and flow meter 9 has been listed as optional, but one ofthem has to be provided for. Both of them may be needed for purposes ofpreparation for practicing the inventive method on a running basisduring supervised. production. The actual withdrawal speed of the ingotis measured by means of a friction contact roll 15 and its outout signal(suitably generated by an tachometer like device or the like), Vrepresents the casting speed.

The purpose of the equipment as described is to provide for certain datawhich are inter-related in such a manner, that the skin thickness s canbe determined.

FIG. 2a is a first graph showing a family of curves wherein a parameterK is plotted against the thickness of s of the skin at the mold exitpoint. K is a dimensionless quantity and defines the'ratio of amount ofheat transferred from the mold wall to the coolant in the upper zone(-a) over the amount of heat transferred from the mold into the coolantas flowing through the lower zone (-b). The variable parameter of thesecurves is total quanity of heat Q fed into the zones, (-a and b), andremoved by the coolant as flowing through duct 8.

FIG. 2bis a second graph wherein the same parameter K is plotted againstsurface temperature T, of the ingot at the mold exit. The parameter ofthe plurality of curves is again Q.

The two graphs of FIG. 2a and 2b have been intentionally plotted withaligned abscissas, so that comparable values of K are alignedhorizontally. One can see, however, that the order of the curvesrepresenting different Qs is reversed.

The relationship of the curves in the two graphs can be demonstratedbest by an example. A particular surface temperature T, at the mold exitintersects with a particular value for K in FIG. 2b and at a curveassociated with a particular parameter Q Or, to state it differently, ifthe ratio of upper to lower heat transfer is K for a total heat flow inboth zones of 0 then the surface temperature of the ingot will be TTaking the same K and using it in the plot of FIG. 2a, then the in- 7tersection of the particular curve for Q with the horizontal lineidentifying K defines the skin thickness s. Skin thickness s, ratio K,and total heat flow Q are related by the following formula:

log Q B (l) log, K A logg log, Q B

wherein A, B, and C are also empirically determined constants. FIG. 2brepresents this formula.

The measuring equipment shown in FIG. 3 can be used to determinedempirically the constants A, A, B, B, C and C. The redundancy ofequation (2) with regard to T, and Q permits readily the determinationof the parameters A, B and C, while measuring the skin thickness withknown techniques permits the determination of parameters A, B and C. Aparticular set of these constant parameters has, of course, validityonly for a speeds V,,, for a particular material that is being cast. Theequations as such and the numerical values for the parameters A, A etccan also be determined by way of mathematical analysis and simulation inconjunction with the measurements.

It is an important aspect that these equations described stationaryconditions, while changes (in time) disturb the relations. Therefor, achange of the value K, for example, due to skin separation in the moldand observed at a particular instant does not change the skin thicknessas emerging from the mold at the same instant. The skin thickness willchange only when the separated skin portion, whose separation caused thechange in K, emerges from the bottom of the mold. Thus, a change in Kwhen detected yields information that a change in skin thickness can beexpected, and that is precisely what one wants to know.

Equation 2 verifies the general statement made above, namely, once theparameters A, B and C have been determined for a particular mold, notall parameters K, T and Q have to be measured for purposes of runningsupervision of the casting process. Having measured either K and T or Kand Q, the respective third parameter can be calculated. One needs Q andK to obtain the skin thickness by operation of equation 1, (FIG. 2a).Please note, that measuring Q and T is feasible as per equation (2) toobtain K, which then could be used in equation (1). In other words FIG.2b and equation (2) could be interpreted as obviating the need formeasuring K, i.e., for separately determining the upper-to-lower heatflow ratio. That however, is a misleading assumption. These equationsdescribe the stationary conditions, and a measured value of K is neededto predict a deviation from the stationary case. Continued utilizationof the equations in a transitory, non-stationary or quasistationary caseis merely an approximation, but if one uses a measured K, thatapproximation suffices to predict a change in s.

The measuring equipment as described has as its principle function,during actual casting, to ascertain the heat flow density into the moldin dependancy upon height, and here particularly in a manner permittingdistinction between the heat flow into the upper mold part and into thelower mold part.

Readings can be taken on a continuous basis. The temperature readings Tand T are indicative of the heat exchange (0,) between mold and coolantin the upper mold half, the readings T and T,, are indicative of theheat exchange (0,.) between mold and coolant in the lower mold half.Coolant flow determination is would be needed for determining theseheats Q,,, Q, individually. However only the ratio Qu/Qe K is actuallyneeded for the relevant determination, and the same amount of coolanttranverses upper and lower zones due to the continuous configuration ofduct 8. Thus, the temperature readings taken with the instruments asdescribed permit directly the derivation of parameter K:

This derived parameter can be calculated on a continuous basis if forexample the measurements are inputted ina process controller. However,known analog circuit networks can simply be used for generating the twodifferences, as well as the ratio to determine K directly as anelectrical (analog) quantity.

This parameter K is referenced in time e.g. against the surfacetemperature of the ingot T, as measured by the pyrometer. Upon using theequation (2) above or a digital function table in a computer, T and Kare used to calculate, also on a running, on-line, real time basis, theparameter Q. Graphically (FIG. 2b) the intersection of T and K defines acurve having a Q as parameter, which is then used in conjunction withFIG. 2a to ascertain skin thickness s.

A computer may use the thus calculated parameter Q and the equation (I)to calculate the skin thickness or to find that value in a look-uptable, stored in the computer as digital representation of the twofamilies of curves of FIGS. 2a and 2b. The value can be outputted forimmediate reading by a (human operator) and/or can be used to controlthe casting process.

One can see, however, that analog circuitry can also be employed, sincethe logarithmic function can be rather simply approximated by linearcircuits such as diodes, and the parameters A, A etc are represented byfixed resistors (having been trimmed to values which are valid for aparticular mold and a particular casting speed). Equations (1), (2) and(3) can, therefor, be represented by a network of linear and non-linearresistance, receiving e.g. voltages representing T T,,-, T T, andproducing an output voltage that is e.g. proportional to logs, or sdirectly, if one uses another non-linear resistance for extracting theoutput from the device.

The skin thickness ascertained in this manner is accurate only for thestationary case. However, in the case of separation of the skin insideof the mold, the parameter K will begin to change before the resultingthinner skin emerges from the bottom of the mold and one can indeed usethe detection of the onset of a change in K to prevent possible disasterby increasing the coolant flow and/or lowering the casting speed.

If one uses analog circuitry for representing actual skin thickness s, areference source providing eg a signal representing desired thickness somay be connected in opposision to the source providing themeasured/calculated signal representing s, and an error signal may beproduced and used further and/or indicated e.g. when a threshold valuefor such an error is exceeded.

The particular mode of practicing the invention with a common coolingduct for both zones, is somewhat simpler in that it is not necessary tomeasure the coolant flow to obtain any other values On, Qe and Q.Rather, Q is determined on the basis of the surface temperature as perequation (2). The operation as such is, of course, valid only for aconstant casting speed. However, the measurements and calculatedresults, either K process, separately for the upper half and the lowerhalf of the mold. The two quantities are respectively denoted Q0 and Onand their ratio is again the parameter K. While 00 +Qu Q, and equation(1) yields the skin thickness.

The example above shows a common flow path for the coolant in upper andlower mold portions, and upper and lower heat flows are distinguished onthe basis of temperature readings in the different portions of thecoolant flow. One can, however, construct the mold to have differentcoolant circulations in upper and lower mold portions, with separateinflow and discharge paths. However, in that case it is essential tometer flow quantities in both circulators to obtain accurate informationon the heat flow quotient K as well as on Q.

Using a single circulation as depicted has the advantage that the amountof water flowing through the system does not have to measured. As wasexplained, Q per se can be treated as a redundant parameter as far asmeasurements is concerned, and for exactly equal water flow in upper andlower mold zones (as is the case in FIG. 3), the ratio K does no longercontain the water quantity but is reduced to the quotient of temperaturedifferentials, K (T T )/(T T However separate cooling flows in lower andlower zones may be of advantage for the cooling process as such, sincethe central mold wall region is cooled with fresh, cool coolant. In thiscase, one needs to meter the coolant flow as the ratio K will no longerbe represented by the ratio of temperature differentials as per equation(3). Rather one has to form K Qu/Qe, with Qu and Qe being separatelydetermined in the two vertically, stacked coolant circulations.

The invention is not limited to the embodiments described above but allchanges and modifications thereof not constituting departures from thespirit and scope of the invention are intended to be included.

We claim:

1. In a method for providing for an anticipating indication of weakeningof the skin of a continuously cast ingot where emerging from the bottomof a liquid cooled mold, and wherein a location of the mold wall hasbeen determined which is particularly prone to exhibit separation of theingot skin as formed adjacently thereof during the casting, theimprovement comprising the steps of:

measuring the heat transfer taking place in an upper portion of the moldat said location, well above the bottom thereof and of a widthconsiderably smaller than the width of a mold side and providing anelectrical signal representative thereof; and

detecting electrically the onset of deviation of the representation froma normal value indicative of absence of skin weakening or separation. 2.In a method as in claim 1, including the step of measuring separatelythe heat transfer in a portion below said upper portion and of the samewidth and providing an electrical signal representative thereof, andproviding a running indication of the ratio of the two signals asrepresenting the two heat transfers.

3. In a method as in claim 1, and including measuring the heat transferin a second portion of the mold, also well above the bottom thereof andat a mold location displaced for said first mentioned location andproviding an electrical signal representative thereof, and relating thesignal representations of the two heat transfers to each other.

4. In a method of ascertaining the skin thickness of a body solidfyingin a mold wherein liquidous material is added to the mold and the bodyis withdrawn from the bottom of the mold, with solidified skin around astill liquidous core, the mold provided with liquid cooling, comprising:

measuring separately the heats transferred by the body as in contactwith the mold to the coolant in two different, vertically spaced zones,each zone having width considerably smaller than the total circumferenceof the mold, and providing a signal representative of the ratio K of theheats as measured, said signal being time variable as said body is beingwithdrawn in dependance upon any variations of any said heat transfers;providing a signal representation for the total heat transfer Q to thecoolant in said two zones; and

indirectly measuring the skin thickness s to be expected at the moldexit by processing said signals on the basis of the functional relationlog, K=A log;

wherein A, B and C are empirically ascertained pa- I rameters for themold.

5. In a method as in claim 4, wherein the heat transfer Q is measuredindirectly, by detecting the surface temperature T, of the body whenwithdrawn from the mold, and by providing the signal representing Q onthe basis of the relation tion, the coolant flowing vertically down inthe mold wall, the temperature being measured in the coolant flow path.

8. Method as in claim 7, wherein the two zones extend respectively inupper and lower half of the mold, the heat ratio detecting stepincluding measuring the temperature T of the coolant as fed to the moldin an upper wall portion thereof, the temperature T,, of the coolant aswithdrawn from the mold in a lower wall portion thereof, and thetemperature T of the coolant in a central wall portion of the mold, andproviding the signal representing K by forming in a measuring circuitthe ratio T T T T on the basis of the separate measuring of saidtemperatures.

9. Method as in claim 8, wherein the common circulation includes avertical duct, the temperatures being detected by using thermo-elementsprojecting into the duct.

10. Method as in claim 4, wherein upper and lower mold wall portions arecooled by separate coolant circulations, the temperature in the centralwall portion being measured separately for the two circulations, once inthe in-flow for the cooling of the lower mold wall portion and once inthe out-flow from cooling the upper mold wall portion.

11. In a method of ascertaining the skin thickness of a body solidifyingin a mold wherein liquidous material is added to the mold and the bodyis withdrawn from the bottom of the mold, with solidified skin around astill liquidous core, the mold provided with liquid cooling, comprising:

monitoring the heat transferred by the ingot to the mold inan upperportion of the mold and for a width portion thereof considerably smallerthan the total circumference of the mold, and providing a signalrepresentative thereof;

monitoring the heat transferred by the ingot to the mold in a lowportion of the mold and for a width portion similar to said width andproviding a signal representative thereof;

monitoring concurrently the total amount of heat Q transferred in bothsaid portions, said monitoring steps including measuring temperature ofthe cooling liquid at various points in said portions as well as therate of flow of said coolant as effective in said portions; and

processing said signals and a signal representation of the quantity Q toprovide an indication of skin thickness s to be expected on the basis ofthe functional relation log,K=A'log,s-l-

wherein A, B' and C are empirically ascertained parameters for the mold.

12. In a method of ascertaining the skin thickness of a body solidifyingin a mold wherein liquidous material is added to the mold and the bodyis withdrawn from the bottom of the mold, with solidified skin around astill liquidous core, the mold provided with liquid cooling, comprising:

monitoring the vertical temperature differential of the cooling liquidin an upper portion of the mold and for a width portion thereofconsiderably smaller than the total circumference of the mold;monitoring the vertical temperature differential of the cooling liquidin a lower portion of the mold and for a width portion also considerablysmaller than the total circumference of the mold; providing a signalrepresentation of the ratio K of said temperature differentials, and ona running basis; measuring the surface temperature T of the body aswithdrawn from the mold; and providing a signal representation of skinthickness by processing said ratio signal K and a signal representingmeasured surface temperature, on the basis of the two functionalrelations log Q B wherein A, A, B, B, C and C are empirically determinedconstants and Q is an auxiliary quantity.

1. In a method for providing for an anticipating indication of weakeningof the skin of a continuously cast ingot where emerging from the bottomof a liquid cooled mold, and wherein a location of the mold wall hasbeen determined which is particularly prone to exhibit separation of theingot skin as formed adjacently thereof during the casting, theimprovement comprising the steps of: measuring the heat transfer takingplace in an upper portion of the mold at said location, well above thebottom thereof and of a width considerably smaller than the width of amold side and providing an electrical signal representative thereof; anddetecting electrically the onset of deviation of the representation froma normal value indicative of absence of skin weakening or separation. 2.In a method as in claim 1, including the step of measuring separatelythe heat transfer in a portion below said upper portion and of the samewidth and providing an electrical signal representative thereof, andproviding a running indication of the ratio of the two signals asrepresenting the two heat transfers.
 3. In a method as in claim 1, andincluding measuring the heat transfer in a second portion of the mold,also well above the bottom thereof and at a mold location displaced forsaid first mentioned location and providing an electrical signalrepresentative thereof, and relating the signal representations of thetwo heat transfers to each other.
 4. In a method of ascertaining theskin thickness of a body solidfying in a mold wherein liquidous materialis added to the mold and the body is withdrawn from the bottom of themold, with solidified skin around a still liquidous core, the moldprovided with liquid cooling, comprising: measuring separately the heatstransferred by the body as in contact with the mold to the coolant intwo different, vertically spaced zones, each zone having widthconsiderably smaller than the total circumference of the mold, andproviding a signal representative of the ratio K of the heats asmeasured, said signal being time variable as said body is beingwithdrawn in dependance upon any variations of any said heat transfers;providing a signal representation for the total heat transfer Q to thecoolant in said two zones; and indirectly measuring the skin thickness sto be expected at the mold exit by processing said signals on the basisof the functional relation
 5. In a method as in claim 4, wherein theheat transfer Q is measured indirectly, by detecting the surfacetemperature To of the body when withdrawn from the mold, and byproviding the signal representing Q on the basis of the relation 6.Method as in claim 4 characterized by carrying out similar steps indifferent areas of the mold spaced apart along the circumference, andcomparing the resulting skin thickness values with each other.
 7. Methodas in claim 4, wherein upper and lower mold wall portions are cooled bya common circulation, the coolant flowing vertically down in the moldwall, the temperature being measured in the coolant flow path.
 8. Methodas in claim 7, wherein the two zones extend respectively in upper andlower half of the mold, the heat ratio detecting step includingmeasuring the temperature TE of the coolant as fed to the mold in anupper wall portion thereof, the temperature TA of the coolant aswithdrawn from the mold in a lower wall portion thereof, and thetemperature TM of the coolant in a central wall portion of the mold, andproviding the signal representing K by forming in a measuring circuitthe ratio TM -TE / TA - TM on the basis of the separate measuring ofsaid temperatures.
 9. Method as in claim 8, wherein the commoncirculation includes a vertical duct, the temperatures being detected byusing thermo-elements projecting into the duct.
 10. Method as in claim4, wherein upper and lower mold wall portions are cooled by separatecoolant circulations, the temperature in the central wall portion beingmeasured separately for the two circulations, once in the in-flow forthe cooling of the lower mold wall portion and once in the out-flow fromcooling the upper mold wall portion.
 11. In a method of ascertaining theskin thickness of a body solidifying in a mold wherein liquidousmaterial is added to the mold and the body is withdrawn from the bottomof the mold, with solidified skin around a still liquidous core, themold provided with liquid cooling, comprising: monitoring the heattransferred by the ingot to the mold inan upper portion of the mold andfor a width portion thereof considerably smaller than the totalcircumference of the mold, and providing a signal representativethereof; monitoring the heat transferred by the ingot to the mold in alow portion of the mold and for a width portion similar to said widthand providing a signal representative thereof; monitoring concurrentlythe total amount of heat Q transferred in both said portions, saidmonitoring steps including measuring temperature of the cooling liquidat various points in said portions as well as the rate of flow of saidcoolant as effective in said portions; and processing said signals and asignal representation of the quantity Q to provide an indication of skinthickness s to be expected on the basis of the functional relation 12.In a method of ascertaining the skin thickness of a body solidifying ina mold wherein liquidous material is added to the mold and the body iswithdrawn from the bottom of the mold, with solidified skin around astill liquidous core, the mold provided with liquid cooling, comprising:monitoring the vertical temperature differential of the cooling liquidin an upper portion of the mold and for a width portion thereofconsiderably smaller than the total circumference of the mold;monitoring the vertical temperature differential of the cooling liquidin a lower portion of the mold and for a width portion also considerablysmaller than the total circumference of the mold; providing a signalrepresentation of the ratio K of said temperature differentials, and ona running basis; measuring the surface temperature To of the body aswithdrawn from the mold; and providing a signal representation of skinthickness by processing said ratio signal K and a signal representingmeasured surface temperature, on the basis of the two functionalrelations